Extended exposure to microgravity (µG) is known to reduce strength in weight-bearing muscles, and was also reported to reduce respiratory muscle strength. Short duration exposure to µG reduces vital capacity (VC), a surrogate measure for respiratory muscle strength, for the first few days, with little change in VO₂, ventilation or end-tidal partial pressures. Accordingly we measured VC, maximum inspiratory and expiratory pressures (MIP and MEP), and indices of pulmonary gas exchange in 10 normal subjects (9M, 1F, 39-52 yrs) who lived on the International Space Station for 130-196 days in a normoxic, normobaric atmosphere. Subjects were studied 4 times in the standing and supine postures preflight at sea-level (1G), ~ monthly in µG, and multiple times postflight. VC in µG was essentially unchanged compared to preflight standing (5.28±0.08 liters [mean±SE], n=187; 5.24±0.09, n=117, respectively; p=0.03) and considerably greater than that measured supine in 1G (4.96±0.10, n=114, p<0.001). There was a trend for VC to decrease after the first 2 months of µG, but there were no changes postflight. Maximum respiratory pressures in µG were generally intermediate to those standing and supine in 1G, and importantly showed no decrease with time spent in µG. O₂ uptake and CO₂ production were reduced (~12%) in extended µG, but inhomogeneity in the lung was not different compared with short-duration exposure to µG. The results show that VC is essentially unchanged and respiratory muscle strength maintained during extended exposure to µG, and metabolic rate is reduced.